1. Field of the Invention
This invention relates generally to digital broadcasting. More particularly, it relates to a reliable and robust multi-stream hybrid AM In-Band On-Channel (IBOC) system (HIA) and technique.
2. Background of Related Art
Various methods of broadcasting signals are known. For instance, FM and AM radio are known broadcast techniques. Moreover, In-Band On-Channel (IBOC) digital broadcast systems (more recently also referred to as hybrid IBOC systems) have been proposed for both AM and FM bands. For instance, in AM IBOC systems, both analog and digital audio are transmitted in the same allocated frequency band (i.e., channel).
In conventional AM radio broadcasts, each radio station is assigned a particular frequency band (i.e., channel) for transmission of analog audio signals. While a frequency band of 20 KHz is typically assigned to each channel, only the center 10 KHz is used to provide side band tolerance and isolation between adjacent channels to avoid adjacent channel interference.
FIG. 2 shows a conventional AM channel 400 containing an analog audio signal 410.
In particular, the analog audio signal 410 is ideally 10 KHz wide and centered about the assigned center frequency. Of course, environmental conditions may cause the actual center frequency fc to vary from the assigned center frequency, possibly interfering with information contained in an adjacent channel. To help avoid adjacent channel interference, the signal transmitted in the AM channel 400 outside of the center 10 KHz is typically required to be attenuated, e.g., xe2x88x9225 dB with respect to the analog audio signal 410 transmitted at the center frequency fc.
More recently, it has been desired to include digital information within and along with conventional AM frequency channels, e.g., 20 KHz frequency bands. Embedding one or more digital data streams within an analog AM channel provides for multiple uses of already crowded frequency spectrum, e.g., for non-commercial broadcasts such as cellular phone transmissions, as well as for commercial broadcasts such as digital television (DTV) or radio services such as AM radio. The digital information streams allow broadcast technology to advance capacity beyond that provided by conventional analog AM broadcast technology, while the analog content continues to allow conventional use of the AM channel allowing backwards compatibility for legacy systems.
One proposed method for embedding multiple digital data streams in a conventional AM channel is shown in FIG. 3.
In particular, in FIG. 3, a typical AM channel 500 includes a host AM analog audio signal 510 together with two digital side band regions 502, 508 and an attenuated region 504 about the center frequency. The upper side band region 502 and the lower side band region 508 are desirably attenuated as required with respect to the level of the AM host analog signal 510. Accordingly, a digital data stream can be transmitted in one, two or all three available digital regions 502, 504 and/or 508 as necessary, all embedded together with a conventional AM audio signal 510.
Conventional systems functioning as that shown in FIG. 3 utilize side bands 502 and 508 for digital transmission. In this spectral area, there is no overlap with the analog host 510. As such, frequency division multiplexing separates analog and digital information.
Because of the narrow band nature of the AM bandwidth, these two side bands 502, 508 do not provide sufficient throughput for typical devices, e.g., a digital audio codec. As such, techniques were sought to include digital transmission in the region occupied by the analog carrier 510. Thus, techniques which take advantage of the one-dimensional nature of the AM analog scheme were sought.
The conventional systems functioning as shown in FIG. 3 allow for digital and analog transmission to co-exist using the same frequencies by placing complementary carriers in quadrature with the analog signal. With the symbols A and their conjugates A* transmitted as shown in FIG. 3, a receiver can retrieve the real part of the transmitted data by adding the symbol with its conjugate. Conversely, the receiver can retrieve the imaginary part of the transmitted data by subtraction. Naturally, since one modulation dimension is supporting the analog host, only one of the two digital modulation dimensions can be detected.
Furthermore, if one of the two bands (A or A*) is severely impaired, retrieval of the transmitted data using addition or subtraction of A and A* will not produce the correct real part of the transmitted data symbol, since the imaginary part cannot be properly canceled by its conjugate. Thus, impairment of either one of the two bands 507, 506 renders both bands 507, 506 essentially useless.
Thus, there is a need for an improved and more robust technique and apparatus for transmitting one or more digital data streams in a conventional AM channel, particularly in the face of adjacent channel interference.
In accordance with the principles of the present invention, in addition to conventional use of frequency division multiplexed upper and lower side bands, the spectral area occupied by the analog host will be utilized by applying a one-dimensional modulation, e.g., pulse amplitude modulation (PAM) as opposed to the conventional use of a two-dimensional modulation, e.g., quadrature amplitude modulation (QAM).
Furthermore, the analog and digital information within the analog host bandwidth may be combined in quadrature to keep the signals orthogonal. Thus, should one side band under the analog carrier be deteriorated or obliterated by adjacent channel interference, the other side band under the analog carrier can still provide useful data and hence better digital audio codec performance.